Because that's where the businesses and business connections are. It's sort of a chicken-and-egg problem. NYC & SV are successful business hubs because they are successful business hubs. Sure, they both had to start somewhere, there's no guarantee of their continued dominance, and that's not to say other business hubs can't come into their own, but it's going to be an uphill battle for new locales wanting to attract talent.

The pandemic & work-from-home made people realize that these hubs aren't really as crucial to business success as people thought they were, but there's still a lot of institutional inertia (not to mention shitty companies requiring employees to return to office for no real reason aside from managerial power trips).